The purpose of this project is the development of a non-invasive method of measuring tissue blood flow by analysis of the spectrum of doppler scattered laser light. A prototype instrument has been developed which is capable of monitoring blood flow in skin and the surface of internal organs. Experiments have shown excellent results in monitoring the effects of pharmacologic agents on the blood flow of rat renal cortex, measured continuously in time, in mapping areas of infarction on the cerebral cortex of the rhesus monkey, and measuring blood flow in human skin. The peripheral vascular changes of hemorrhagic shock in rats have been monitored to study the usefulness of the instrument as an early indicator of circulatory failure. Fiber optic waveguides have been incorporated into the instrument and greatly increase the flexibility of the instrument and its suitability for clinical use.